At present, a variety of technologies are suggested about an electromagnetic conversion unit where a permanent magnet and a vibrating membrane are combined. For example, an electromagnetic conversion unit described in Patent Document 1 includes a permanent magnet board, a vibrating membrane disposed at a position opposed to the permanent magnet board, and a buffer member interposed between the permanent magnet board and the vibrating membrane. In the permanent magnet board, belt-like magnetic poles different from each other in magnetic polarity are formed alternately at a certain spacing. Further, in the vibrating membrane, a coil with a meandering conductor pattern is formed at a position opposed to a so-called “neutral zone of magnetization” that is a gap between the magnetic poles different from each other in magnetic polarity.
By those arrangement, when a current (audio signal) flows through the coil of the vibrating membrane, the coil and the multipole magnetized pattern of the permanent magnet board are electromagnetically coupled to each other to generate audio vibration in the vibrating membrane according to Fleming's rule. Further, the permanent magnet board, the vibrating membrane, and a buffer member are enclosed by a metallic frame to be attached to a speaker casing, and the sound wave generated by the vibration is emitted through an emitting hole provided through the permanent magnet board and the metallic frame to reproduce audio data.    Patent Document 1: JP-A-09-331596 (JP-A-1997-331596)
The conventional electromagnetic conversion unit is arranged as described above, and thus it is necessary to further provide a permanent magnet board at a position opposed to the vibrating membrane so as to hold the vibrating membrane from two directions by sandwiching the membrane between the permanent magnet boards. There is a problem that, when the thickness of the permanent magnet board is increased, the total thickness of the electromagnetic conversion unit increases, which can make it impossible to be built in a thin electronic apparatus. Therefore, it is required to use a magnet having a high maximum energy product (BHmax) such as a neodymium iron boron magnet in order to obtain a magnetic flux density required for driving the vibrating membrane without increasing the thickness of the permanent magnet board. Thus, there is a problem that the cost of the magnetic circuit can be boosted.
The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an electromagnetic conversion unit capable of obtaining a magnetic flux density necessary for driving a vibrating membrane without increasing the total thickness of the electromagnetic conversion unit even when the magnetic circuit is built with a magnet having a low maximum energy product.